Non-linear diffusion during phase separation and symplectite formation in alkali feldspar: an experimental and modelling approach (TP8)

碱长石相分离和辛辉石形成过程中的非线性扩散：实验和建模方法 (TP8)

• 批准号: 33871937
• 负责人: Professor Dr. Rainer Abart
• 金额: --
• 依托单位: Helmholtz-Zentrum Potsdam - Deutsches GeoForschungsZentrum (GFZ)
• 依托单位国家: 德国
• 项目类别: Research Units
• 财政年份: 2007
• 资助国家: 德国
• 起止时间: 2006-12-31 至 2014-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/33871937?language=en
• 关键词: Non; linear; diffusion; during; phase

In the precursor project we studied microstructure evolution during spinodal decomposition of alkali feldspar in experiment and theoretically. With the successor project we want to extend this study by (i) addressing nucleation-growth as an alternative reaction pathway for exsolution and (ii) investigating in detail Na+/K+ interdiffusion in alkali feldspar. (i) To induce exsolution by nucleation and growth, isothermal anneals will be done at conditions where the homogeneous alkali feldspar is metastable. Experiments will be done at high pressure in the 1 to 2 GPa range, where the metastable region in the alkali feldspar binary extends to high temperature, and the overall reaction kinetics is fast. The exsolution microstructure and microchemistry will be investigated using electron beam microanalytical techniques with high spatial resolution including FEG-SEM, FEG-EMP and TEM combined with FIB sample preparation. The numerical model that was developed in the precursor project will be complemented with a nucleation module to extract process parameters from comparing modelled and observed microstructures and phase compositions. (ii) From experiments in the precursor project we find that the geometries of concentration profiles produced from Na+/K+ exchange between alkali feldspar and NaCl-KCl salt melt deviate from what is expected for simple interdiffusion. Exchange fronts propagating through the crystal show a sharpness, which depends on the composition difference between the pristine and the exchanged domains of the grain but does not change with time. We hypothesize that this may be due to strain occurring at the interface between exchanged and pristine crystal due to lattice mismatch (coherence-strain). The mechanisms by which this occurs are, however, elusive. Solving the question of coherence strain effects on cation interdiffusion is fundamental for the understanding of interdiffusion and exsolution in mineral systems. We suggest doing cation exchange experiments using different starting materials including isotopically labelled KCl with well-defined geometry. The microchemistry, defect density and lattice distortion at the exchange fronts will be investigated using high resolution analytical techniques, and conceptual models for the influence of coherency strain on Na+/K+ interdiffusion in alkali feldspar will be developed and parmeterized.

在前期项目中，我们从实验和理论上研究了碱长石调幅分解过程中的显微结构演化。在后续项目中，我们希望通过以下方式扩展这项研究：（i）解决成核-生长作为出溶的替代反应途径，以及（ii）详细研究碱性长石中的Na+/K+相互扩散。(i)为了通过成核和生长诱导出溶，将在均质碱长石为亚稳的条件下进行等温退火。实验将在1至2 GPa范围内的高压下进行，其中碱长石二元中的亚稳区延伸到高温，并且整体反应动力学快速。利用高空间分辨率的电子束微区分析技术，包括FEG-SEM、FEG-EMP和TEM，结合FIB样品制备，研究了出溶体的微观结构和微化学。在前体项目中开发的数值模型将补充成核模块，以通过比较建模和观察到的微观结构和相组成来提取工艺参数。(ii)从实验中的前体项目，我们发现，从碱长石和NaCl-KCl盐熔体之间的Na+/K+交换产生的浓度分布的几何形状偏离简单的相互扩散所预期的。通过晶体传播的交换前沿显示出尖锐性，这取决于原始和交换的晶粒域之间的组成差异，但不随时间而改变。我们推测，这可能是由于应变发生在交换和原始晶体之间的界面处，由于晶格失配（相干应变）。然而，发生这种情况的机制是难以捉摸的。解决相干应变对阳离子互扩散的影响问题是理解矿物系统中的互扩散和出溶的基础。我们建议使用不同的起始材料进行阳离子交换实验，包括具有明确几何结构的同位素标记的KCl。微观化学，缺陷密度和晶格畸变的交换前沿将使用高分辨率的分析技术进行调查，和概念模型的一致性应变的影响Na+/K+相互扩散的碱性长石将开发和parmeterized。